diff --git a/compiler/rustc_hir_typeck/src/callee.rs b/compiler/rustc_hir_typeck/src/callee.rs
index f2c58ee27029b..4c03ff197a7ba 100644
--- a/compiler/rustc_hir_typeck/src/callee.rs
+++ b/compiler/rustc_hir_typeck/src/callee.rs
@@ -792,8 +792,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 tcx.consts.false_
             }
             Some(hir::ConstContext::ConstFn) => {
-                let args = ty::GenericArgs::identity_for_item(tcx, context);
-                args.host_effect_param().expect("ConstContext::Maybe must have host effect param")
+                ty::GenericArgs::identity_for_item(tcx, callee_did).const_at(host_effect_index)
             }
             None => tcx.consts.true_,
         };
diff --git a/compiler/rustc_middle/src/ty/generic_args.rs b/compiler/rustc_middle/src/ty/generic_args.rs
index 72390e4bbb03b..a861af4785945 100644
--- a/compiler/rustc_middle/src/ty/generic_args.rs
+++ b/compiler/rustc_middle/src/ty/generic_args.rs
@@ -11,7 +11,6 @@ use rustc_errors::{DiagnosticArgValue, IntoDiagnosticArg};
 use rustc_hir::def_id::DefId;
 use rustc_macros::HashStable;
 use rustc_serialize::{self, Decodable, Encodable};
-use rustc_span::sym;
 use rustc_type_ir::WithCachedTypeInfo;
 use smallvec::SmallVec;
 
@@ -452,10 +451,6 @@ impl<'tcx> GenericArgs<'tcx> {
         tcx.mk_args_from_iter(self.iter().take(generics.count()))
     }
 
-    pub fn host_effect_param(&'tcx self) -> Option<ty::Const<'tcx>> {
-        self.consts().rfind(|x| matches!(x.kind(), ty::ConstKind::Param(p) if p.name == sym::host))
-    }
-
     pub fn print_as_list(&self) -> String {
         let v = self.iter().map(|arg| arg.to_string()).collect::<Vec<_>>();
         format!("[{}]", v.join(", "))
diff --git a/compiler/rustc_middle/src/ty/instance.rs b/compiler/rustc_middle/src/ty/instance.rs
index 2b75f6c4e8f23..8450ad13677ff 100644
--- a/compiler/rustc_middle/src/ty/instance.rs
+++ b/compiler/rustc_middle/src/ty/instance.rs
@@ -341,6 +341,11 @@ impl<'tcx> Instance<'tcx> {
             !args.has_escaping_bound_vars(),
             "args of instance {def_id:?} not normalized for codegen: {args:?}"
         );
+        if cfg!(debug_assertions) {
+            super::tls::with(|tcx| {
+                assert_eq!(tcx.generics_of(def_id).count(), args.len());
+            });
+        }
         Instance { def: InstanceDef::Item(def_id), args }
     }
 
diff --git a/compiler/rustc_mir_transform/src/shim.rs b/compiler/rustc_mir_transform/src/shim.rs
index e9895d97dfefa..d6ac1555b706c 100644
--- a/compiler/rustc_mir_transform/src/shim.rs
+++ b/compiler/rustc_mir_transform/src/shim.rs
@@ -661,7 +661,7 @@ fn build_call_shim<'tcx>(
         // Create substitutions for the `Self` and `Args` generic parameters of the shim body.
         let arg_tup = Ty::new_tup(tcx, untuple_args);
 
-        (Some([ty.into(), arg_tup.into()]), Some(untuple_args))
+        (Some([ty.into(), arg_tup.into(), tcx.consts.true_.into()]), Some(untuple_args))
     } else {
         (None, None)
     };
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index a76272e9d092d..f3daaacf956a7 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -271,7 +271,7 @@ pub fn closure_trait_ref_and_return_type<'tcx>(
         TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
         TupleArgumentsFlag::Yes => Ty::new_tup(tcx, sig.skip_binder().inputs()),
     };
-    let trait_ref = ty::TraitRef::new(tcx, fn_trait_def_id, [self_ty, arguments_tuple]);
+    let trait_ref = ty::TraitRef::new(tcx, fn_trait_def_id, [ty::GenericArg::from(self_ty), arguments_tuple.into(), tcx.consts.true_.into()]);
     sig.map_bound(|sig| (trait_ref, sig.output()))
 }
 
diff --git a/library/core/src/ops/function.rs b/library/core/src/ops/function.rs
index 20f0bba4c13b6..667f006f2dbea 100644
--- a/library/core/src/ops/function.rs
+++ b/library/core/src/ops/function.rs
@@ -72,8 +72,8 @@ use crate::marker::Tuple;
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-// FIXME(effects) #[const_trait]
-pub trait Fn<Args: Tuple>: FnMut<Args> {
+#[const_trait]
+pub trait Fn<Args: Tuple>: ~const FnMut<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
     extern "rust-call" fn call(&self, args: Args) -> Self::Output;
@@ -159,8 +159,8 @@ pub trait Fn<Args: Tuple>: FnMut<Args> {
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-// FIXME(effects) #[const_trait]
-pub trait FnMut<Args: Tuple>: FnOnce<Args> {
+#[const_trait]
+pub trait FnMut<Args: Tuple>: ~const FnOnce<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
     extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
@@ -238,7 +238,7 @@ pub trait FnMut<Args: Tuple>: FnOnce<Args> {
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-// FIXME(effects) #[const_trait]
+#[const_trait]
 pub trait FnOnce<Args: Tuple> {
     /// The returned type after the call operator is used.
     #[lang = "fn_once_output"]
diff --git a/tests/rustdoc/rfc-2632-const-trait-impl.rs b/tests/rustdoc/rfc-2632-const-trait-impl.rs
index 7f56b2ffeb8ad..e9391d063a044 100644
--- a/tests/rustdoc/rfc-2632-const-trait-impl.rs
+++ b/tests/rustdoc/rfc-2632-const-trait-impl.rs
@@ -7,7 +7,7 @@
 // not remove this test.
 //
 // FIXME(effects) add `const_trait` to `Fn` so we use `~const`
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, effects)]
 #![crate_name = "foo"]
 
 use std::marker::Destruct;